1. Field of the Invention
The present invention relates to a lithographic apparatus and a method of manufacturing a device.
2. Brief Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
It is known in the art to provide a lithographic apparatus with a chuck assembly for supporting the substrate and/or patterning device (e.g. a mask or reticle) during operations. A typical prior art chuck assembly includes a chuck provided with a clamp, which for example uses electrostatic or vacuum forces. By the clamp, the substrate or patterning device can be clamped on the chuck, inter alia to retain the flatness of the clamped object. The chuck, in turn, is supported by a frame with respect to other parts of the lithographic apparatus.
However, during the lithographic process, the chuck may be subjected to temperature changes, due to heat transferred from the substrate, for example. The change in temperature affects the accuracy of the projection of the pattern on the substrate, because deformations of the chuck and the thermal changes are, at least to some extent, transferred to the substrate. Even small changes in temperature (e.g. changes smaller than 0.05 K), and in particular thermal inhomogenities of the chuck, give rise to local thermal expansions or shrinkage of the chuck in the order of the suitable precision of the projection, typically in the range of 1 micron to several nanometers.
To reduce the thermal changes in the chuck, it is generally known to provide a thermal system to the chuck which is capable of removing heat from the chuck, to maintain the chuck at a constant temperature level.
From U.S. Pat. No. 5,413,167, for example, a wafer holding block is known which comprises a wafer chuck. The wafer chuck is formed with crossing grooves communicating with a vacuum pump, for vacuum attraction of a wafer. The wafer chuck is fixed on a fine motion stage, for fine alignment of the wafer to a mask. The fine motion stage is provided on a central portion of a support table which is fixed on a rough motion stage for rough alignment of the wafer to the mask. The wafer chuck is made of an aluminum material with a high thermal conductivity. A heat exchanger is fixedly provided on the rough motion stage. The heat exchanger has a passageway coupled with two cooling water pipes. Two flexible heat pipes are used to provide heat transmission between the wafer chuck and the heat exchanger. The heat pipes have opposite end portions made of an aluminum material with good heat conductivity, and a central pipe portion made of a resin material with heat resistivity and a wick wetted with an operative liquid. When operated, heat is transported from the wafer chuck to the heat exchanger via the flexible heat pipes.
From U.S. Pat. No. 6,215,642 a vacuum compatible deformable electrostatic chuck is known. The chuck has a high thermal conductivity. The chuck includes a membrane having a layer of dielectric material, a layer of metallic film and a layer of semiconductor material. Struts and a rim are formed on the layer of semiconductor material. The rim is formed on the periphery of the layer of semiconductor material. The rim and struts contact a surface of a supporting structure and form a hollow area between the membrane and the supporting structure in which a coolant gas is circulated. The supporting structure has gas manifold holes to connect the hollow area with a source of coolant gas. Heat can be transferred away from the chuck via the coolant gas. Between the gas filled hollow area and a backside of the supporting structure, the supporting structure further has a hollow portion in which a coolant liquid is circulated.
From U.S. Pat. No. 5,220,171 a wafer holding block is known, which comprises a wafer chuck. The wafer chuck is formed with crossing grooves communicated with a vacuum pump, for vacuum attraction of a wafer. The wafer chuck is fixed on a fine motion stage, for fine alignment of the wafer to a mask. The fine motion stage is provided on a central portion of a support table which is fixed on a rough motion stage for rough alignment of the wafer to the mask. The wafer chuck is made of an aluminum material with a high thermal conductivity. The chuck has a reduced pressure inside space. A wick wetted with an operative liquid is adhered to the inside wall of the space. A cooling plate with cooling water passageways is interposed between the fine-motion stage and the wafer chuck. By circulation of cooling water through the cooling plate, heat can be transferred from the chuck and the cooling surface of the wafer chuck can be maintained at a temperature of about 20 degrees Celsius.
A drawback of the chuck assemblies known from these prior art documents is that the position of the chuck assembly, and the substrate, with respect to the beam of radiation is subject to vibrations and other distortions originating from the component on which the chuck is fixated. Thereby, the accuracy of the patterning is affected. The distortions are especially disadvantageous in view of current and expected trends in the accuracy requirements due to the decreasing dimensions of the structures projected onto the substrate.